Flow rate measurement for industrial sensing applications using unsteady pressures

ABSTRACT

Flow rate measurement system includes two measurement regions  14,16  located an average axial distance ΔX apart along the pipe  12 , the first measurement region  14  having two unsteady pressure sensors  18,20 , located a distance X 1  apart, and the second measurement region  16 , having two other unsteady pressure sensors  22,24,  located a distance X 2  apart, each capable of measuring the unsteady pressure in the pipe  12 . Signals from each pair of pressure sensors  18,20  and  22,24  are differenced by summers  44,54 , respectively, to form spatial wavelength filters  33,35 , respectively. Each spatial filter  33,35  filters out acoustic pressure disturbances P acoustic  and other long wavelength pressure disturbances in the pipe  12  and passes short-wavelength low-frequency vortical pressure disturbances P vortical  associated with the vortical flow field  15 . The spatial filters  33,35  provide signals P as1 ,P as2  to band pass filters  46,56  that filter out high frequency signals. The P vortical  -dominated filtered signals P asf1 ,P asf2  from the two regions  14,16  are cross-correlated by Cross-Correlation Logic  50  to determine a time delay τ between the two sensing locations  14,16  which is divided into the distance ΔX to obtain a convection velocity U c (t) that is related to an average flow rate of the fluid (i.e., one or more liquids and/or gases) flowing in the pipe  12 . The invention may also be configured to detect the velocity of any desired inhomogeneous pressure field in the flow. The invention may also be combined with an instrument, an opto-electronic converter and a controller in an industrial process control system.

CROSS REFERENCES TO RELATED APPLICATIONS

[0001] This application is an continuation-in-part of commonly ownedco-pending U.S. patent applications, Ser. No., 09/346,607, entitled“Flow Rate Measurement Using Unsteady Pressures”, filed Jul. 2, 1999.

TECHNICAL FIELD

[0002] This invention relates to the measurement of flow rate of a fluidand more particularly to measuring flow rate using unsteady pressuremeasurements for use in industrial sensing applications.

BACKGROUND ART

[0003] An industrial process sensor is typically a transducer thatresponds to a measurand with a sensing element and converts the variableto a standardized transmission signal, e.g., an electrical or opticalsignal, that is a function of the measurand. Industrial process sensorsutilize transducers that include flow measurements of an industrialprocess such as that derived from slurries, liquids, vapors and gassesin refinery, chemical, paper, pulp, petroleum, gas, pharmaceutical,food, mining, minerals and other fluid processing plants. Industrialprocess sensors are often placed in or near the process fluids, or infield applications. Often, these field applications are subject to harshand varying environmental conditions that provide challenges fordesigners of such sensors. Flow measurement is one of the largestsegments of the industrial sensing and instrumentation market.Industries in which flow measurement is prevalent includes petroleum,chemical, pulp, paper, food, and mining and minerals.

[0004] In many industries it is desirable to measure the flow rate of amultiphase fluid. In many industries such as refinery, chemical, paper,pulp, petroleum, gas, pharmaceutical, food, mining, minerals orcomparable industries, for example, it is desirable to measure the flowrate of multiphase fluids, especially fluids having three phases, suchas a constituent, water and gas. It is known also to measure the flowrate of certain fluids (one or more liquids and/or gases) in a pipeusing cross-correlation flow meters. Such meters measure an element ofthe flow that moves or convects with (or is related to) the fluid flow(or a group of fluid particles). The meter measures this element at twolocations separated by a known distance along the flow path and thencalculates the time for such element to move between the two locations.The time delay is determined by a cross-correlation of the two measuredsignals. A velocity is then determined by the distance between themeasurements divided by the time delay. The flow velocity is thenrelated to the flow rate by calibration.

[0005] One such cross-correlation meter that measures flow rate in amultiphase flow is described in U.S. Pat. No. 5,591,922, entitled“Method and Apparatus for Measuring Multiphase Flow”, to Segeral et al,issued Jan. 7, 1997. In that case, a pair of venturis are located apredetermined distance apart which induce a change in flow speed throughthe venturi and a resulting pressure difference (or delta-P) across eachventuri, which are measured. The delta-P pressure signals measured ateach venturi are cross-correlated to determine the time delay which isindicative of the total volume flow rate. However, such a techniquerequires a change in the flow properties (e.g., flow velocity ordensity) at the two measurement points to make the measurement. Also,the delta-P is generated with an area contraction or constriction, andis not a naturally occurring observable characteristic of the fluid.

[0006] Other flowmeters of the prior art include turbine, vortex,electromagnetic and venturi and all have drawbacks and deficienciessolved by the flowmeter of the present invention. For instance allrequire a high level of maintenance and need to be removed from theprocess line wherein the operators need to shut down the manufacturingprocess. The flowmeters of the prior art require electrical wiring andpower that requires enormous cost, safety issues and sometimes miles ofwires. May of the prior art meters use moving parts, such as turbines ordiaphragms. Also, prior art flowmeters such as vortex, turbine andventuri types use obstructions in the flow path that disrupt the flow tovarying degrees. In addition sediment, gumming, plugging, corrosion, anderosion of certain features of the sensing region of the meter canaffect the accuracy of the flowmeter.

[0007] In particular, electromagnetic flowmeters are prone to problemscaused by poor process grounding, and specialized sleeves that prone todamage. Process noise is a problem and can be caused by slurries, highconsistency pulp stock, or upstream chemical additions. Such processnoise can lead to inaccurate flow measurement in these types of priorart flowmeters. In addition, process liquids must have a minimumconductivity that all but eliminates these meters from uses where thefluid is a hydrocarbon . The accuracy and sensitivity can be affected bythe length of cabling for remote transmitters and can be adverselyinfluenced by proximity to other electrical devices Another flow meterof the prior art includes a friction flowmeter such as that set forth inU.S. Pat. No. 6,253,624, titled “Friction Flowmeter” wherein atransducer determines the pressure drop of a fluid flowing along a pipe.The device determines the flow rate of the fluid based on the pressuredrop for a given friction factor of the inside surface of the pipe. Sucha device requires external knowledge of various parameters of the fluid,such as density. In addition, with certain applications the surface ofthe pipe, and the friction factor thereby, would change over time anddecrease the accuracy of the meter in predicting fluid flow rates.

[0008] Typical electronic, or other, transducers of the prior art oftencannot be placed in industrial process environments due to sensitivityto electromagnetic interference, radiation, heat, corrosion, fire,explosion or other environmental factors. It is for these reasons thatfiber optic based sensors are being incorporated into industrial processcontrol environments in increasing number.

SUMMARY OF THE INVENTION

[0009] Objects of the present invention include provision of a systemfor measuring the flow rate (or velocity) of fluid flow in pipes inindustrial fluid processes.

[0010] According to the present invention, an apparatus for use in anindustrial process for measuring a velocity of a fluid moving in a pipe,comprises a first filter which measures a vortical pressure field at afirst axial location along the pipe which provides a first pressuresignal indicative of the vortical pressure field; and a second filterwhich measures the vortical pressure field at a second axial locationalong the pipe which provides a second pressure signal indicative of thevortical pressure field. The invention further comprises a signalprocessor, responsive to the first and the second pressure signals,which provides a velocity signal indicative of a velocity of thevortical pressure field moving in the pipe.

[0011] According further to the present invention, the first and thesecond filters passes wavelengths associated with the vortical pressurefield and not associated with an acoustic pressure field. Accordingfurther to the present invention, the first filter comprises a firstspatial filter; and the second filter comprises a second spatial filter.According still further to the present invention, the vortical pressurefield comprises a homogeneous pressure field. Still further according tothe present invention, the first and the second filters pass wavelengthsassociated with the vortical pressure field and not associated with anacoustic pressure field. According further to the present invention, thespatial filter filters out wavelengths above a predetermined wavelength.Still further according to the present invention, at least one of thepressure sensors comprises a strain gage disposed on a surface of thepipe. Further according to the present invention, the strain gagecomprises a fiber optic strain gage.

[0012] Still further according to the present invention, the signalprocessor comprises logic which calculates a cross-correlation betweenthe first and the second inhomogeneous pressure signals and provides atime delay signal indicative of the time it takes for the vorticalpressure field to move from the first location to the second location.Further according to the present invention, the velocity signal isindicative of the velocity of the fluid moving in the pipe.

[0013] The present invention provides a significant improvement over theprior art by providing a measurement of the average flow rate of fluidflow in a pipe or other conduit (where a fluid is defined as one or moreliquids and/or gases) without requiring a flow restriction in the pipeor any other change in the flow velocity of the fluid.

[0014] The present invention determines a convection velocity bymeasuring unsteady (or dynamic or ac) pressures and extracting thepressure signal indicative of a vortical pressure (or flow) field (orperturbation) which convects at or near the average velocity of thefluid. The vortical pressure field is then used to determine theconvection velocity by cross-correlation techniques, such convectionvelocity being proportional (or approximately equal to) the flow rate ofthe fluid. If needed, the flow rate of the fluid may then be determinedby calibrating the convection velocity to the flow rate.

[0015] The invention may also be used to measure the velocity of anyinhomogeneous flow field, such as gas bubbles, gas slugs, particles, orchunks of material, and its associated pressure field that propagates ina flow provided the spatial filters have a separation within theacceptable coherence length of the flow field to be measured and thesensor spacing within each spatial filter is longer than acharacteristic axial length of the flow field. Also, the invention maybe used to detect different flow rates within the same mixture (e.g.,the flow rate of a vortical pressure field as well as otherinhomogeneous pressure fields).

[0016] Also, the invention may be used with any combination of liquidsand/or gases and may include particles. The invention will also work inany other environment or applications or any other fluids (one or moreliquids and/or gases) or mixtures. The invention will work with any pipeor tube or with any conduit that carries fluid. Also, the invention hasno inherent flow range limitations, and, as such, can measure very lowflow rates and has no maximum flow rate limit. The invention will alsowork if the fluid is flowing in either direction in the pipe. Further,the invention may be used directly on a pipe or on a tube inserted intoa flowing fluid.

[0017] Also, the invention may be combined with a controller and otherdevices and used in an industrial process control system.

[0018] The foregoing and other objects, features and advantages of thepresent invention will become more apparent in light of the followingdetailed description of exemplary embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a schematic block diagram of a velocity measurementsystem, in accordance with the present invention.

[0020]FIG. 2 is a side view of a pipe having two sensors that measure aparameter that convects with the flow in the pipe, in accordance withthe present invention.

[0021]FIG. 3 is a graph of two curves, one from each of the two sensorsof FIG. 2, in accordance with the present invention.

[0022]FIG. 4 is a graph of a cross-correlation between the two curves ofFIG. 3, in accordance with the present invention.

[0023]FIG. 5 is a graph of power spectral density plotted againstfrequency for an unsteady acoustic pressure signal P_(acoustic) andunsteady vortical pressure signal P_(vortical), in accordance with thepresent invention.

[0024]FIG. 6 is a graph of wavelength versus frequency for unsteadyacoustic pressures P_(acoustic) and unsteady vortical pressuresP_(vortical), in accordance with the present invention.

[0025]FIG. 7 is a graph of power spectrum of two unsteady pressures andthe difference between the two pressures, in accordance with the presentinvention.

[0026]FIG. 8 is a graph of a cross-correlation between two of the curvesof FIG. 7, in accordance with the present invention.

[0027]FIG. 9 is a graph of measured velocity against reference velocity,in accordance with the present invention.

[0028]FIG. 10 is a side view of a pipe having three pairs of unsteadypressure sensors spaced axially along the pipe, in accordance with thepresent invention.

[0029]FIG. 11 is a graph of a cross correlation coefficient versus timedelay for the three pairs of sensors, in accordance with the presentinvention.

[0030]FIG. 12 is a graph of measured flow rate against reference flowrate for various different mixtures, in accordance with the presentinvention.

[0031]FIG. 13 is an end view of a pipe showing pressure inside andoutside the pipe, in accordance with the present invention.

[0032]FIG. 14 is a side view of a pipe having optical fiber wrappedaround the pipe at each unsteady pressure measurement location and apair of Bragg gratings around each optical wrap, in accordance with thepresent invention.

[0033]FIG. 15 is a side view of a pipe having optical fiber wrappedaround the pipe at each unsteady pressure measurement location with asingle Bragg grating between each pair of optical wraps, in accordancewith the present invention.

[0034]FIG. 16 is a side view of a pipe having optical fiber wrappedaround the pipe at each unsteady pressure measurement location withoutBragg gratings around each of the wraps, in accordance with the presentinvention.

[0035]FIG. 17 is an alternative geometry of an optical wrap of FIGS.14,15&16, of a radiator tube geometry, in accordance with the presentinvention.

[0036]FIG. 18 is an alternative geometry of an optical wrap of FIGS.14,15&16, of a race track geometry, in accordance with the presentinvention.

[0037]FIG. 19 is a side view of a pipe having a pair of gratings at eachaxial sensing location, in accordance with the present invention.

[0038]FIG. 20 is a side view of a pipe having a single grating at eachaxial sensing location, in accordance with the present invention.

[0039]FIG. 21 is a side view of a pipe having two pairs of pressuresensors where the sensors in each pair are located across the pipe fromeach other, in accordance with the present invention.

[0040]FIG. 22 is an end view of a pipe showing a pair of pressuresensors located at various circumferential spacings from each other, inaccordance with the present invention.

[0041]FIG. 23 is a side view of a pipe having two pairs of pressuresensors where the sensors in each pair are located transversely acrossthe pipe and spaced axially along the pipe from each other, inaccordance with the present invention.

[0042]FIG. 24 is a side view of a pipe having a set of three pressuresensors that form a spatial filter, in accordance with the presentinvention.

[0043]FIG. 25 is a side view of a pipe having an inner tube with axiallydistributed optical fiber wraps for unsteady pressure sensors, inaccordance with the present invention.

[0044]FIG. 26 is a side view of a pipe having an inner tube with axiallydistributed unsteady pressure sensors located along the tube, inaccordance with the present invention.

[0045]FIG. 27 is a side view of a pipe having an inner tube with fouraxially distributed optical fiber wrapped hydrophones located within thetube, in accordance with the present invention.

[0046]FIG. 28 is an end view of a pipe showing a pair of pressuresensors spaced apart from each other within the pipe, in accordance withthe present invention.

[0047]FIG. 29 is a side view of a pipe having a pair of unsteadypressure sensors spaced axially within the pipe, in accordance with thepresent invention.

[0048]FIG. 30 is a side view of a pipe having a pair of unsteadypressure sensors spaced transversely within the pipe, in accordance withthe present invention.

[0049]FIG. 31 is a side view of a pipe having a pair of unsteadypressure sensors axially and radially spaced within the pipe, inaccordance with the present invention.

[0050]FIG. 32 is a side view of a pipe having a set of three pressuresensors that make up two spatial filters, in accordance with the presentinvention.

[0051]FIG. 33 is a schematic drawing of a flow meter in an industrialprocess control system, in accordance with the present invention.

[0052]FIG. 34 is a plan view of alternate geometries for electronicstrain gages in accordance with the present invention.

[0053]FIG. 35 is a side view of a pipe having a spatial filter disposedthereon comprised of electronic strain gages in accordance with thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0054] Referring to FIG. 1, a velocity and flow measurement systemincludes a sensing section 10 along a pipe 12 and a velocity logicsection 40. The pipe (or conduit) 12 has two measurement regions 14,16located a distance ΔX apart along the pipe 12. At the first measurementregion 14 are two unsteady (or dynamic or ac) pressure sensors 18,20,located a distance X₁ apart, capable of measuring the unsteady pressurein the pipe 12, and at the second measurement region 16, are two otherunsteady pressure sensors 22,24, located a distance X₂ apart, capable ofmeasuring the unsteady pressure in the pipe 12. Each pair of pressuresensors 18,20 and 22,24 act as spatial filters to remove certainacoustic signals from the unsteady pressure signals, and the distancesX₁,X₂ are determined by the desired filtering characteristic for eachspatial filter, as discussed hereinafter. The flow measurement system 10of the present invention measures velocities associated with unsteadyflow fields and/or pressure disturbances represented by 15 associatedtherewith relating to turbulent eddies (or vortical flow fields),inhomogeneities in the flow (such as bubbles, slugs, and the like), orany other properties of the flow, fluid, or pressure, having timevarying or stochastic properties that are manifested at least in part inthe form of unsteady pressures. The vortical flow fields are generatedwithin the fluid of the pipe 12 by a variety of non-discrete sourcessuch as remote machinery, pumps, valves, elbows, as well as the fluidflow itself. It is this last source, the fluid flowing within the pipe,that is a generic source of vortical flow fields primarily caused by theshear forces between the fluid and the wall of the pipe that assures aminimum level of disturbances for any fluid piping systems for which thepresent invention takes unique advantage. The flow generated vorticalflow fields generally increase with mean flow velocity and do not occurat any predeterminable frequency. As such, no external discrete vortexgenerating source is required within the present invention and thus mayoperate using passive detection. It is within the scope of the presentthat the pressure sensor spacing may be known or arbitrary and that asfew as two sensors are required if certain information is known aboutthe acoustic properties of the system as will be more fully describedherein below.

[0055] The vortical flow fields 15 are, in general, comprised ofpressure disturbances having a wide variation in length scales and whichhave a variety of coherence length scales such as that described in thereference “Sound and Sources of Sound”, A. P.Dowling et al, HalstedPress, 1983. Certain of these vortical flow fields convect at or near/orrelated to the mean velocity of at least one of the fluids within amixture flowing in a pipe. More specifically, the vortices convect in apredictable manner with reference to the fluids. The vortical pressuredisturbances 15 that contain information regarding convection velocityhave temporal and spatial length scales as well as coherence lengthscales that differ from other disturbances in the flow. The presentinvention utilizes these properties to preferentially selectdisturbances of a desired axial length scale and coherence length scaleas will be more fully described hereinafter. For illustrative purposes,the terms vortical flow field and vortical pressure field will be usedto describe the above-described group of unsteady pressure fields havingtemporal and spatial length and coherence scales described herein.

[0056] The pressures P₁,P₂,P₃,P₄ may be measured through holes in thepipe 12 ported to external pressure sensors or by other techniquesdiscussed hereinafter. The pressure sensors 18,20,22,24 providetime-based pressure signals P₁(t),P₂(t),P₃(t), P₄(t) on lines30,32,34,36, respectively, to Velocity Logic 40 which provides aconvection velocity signal U_(c)(t) on a line 42 which is related to anaverage flow rate U_(f)(t) of the fluid flowing in the pipe 12 (wherefluid may comprise one or more liquids and/or gases; where the gas(es)may be dissolved in the liquid or in free gas form, such as bubbles,slugs, sand, particulates, slurry, etc.), and wherein the fluid mayinclude non-liquid elements therein as will be discussed morehereinafter.

[0057] Also, some or all of the functions within the Velocity Logic 40may be implemented in software (using a microprocessor or computer)and/or firmware, or may be implemented using analog and/or digitalhardware, having sufficient memory, interfaces, and capacity to performthe functions described herein.

[0058] In particular, in the Velocity Logic 40, the pressure signalP₁(t) on the line 30 is provided to a positive input of a summer 44 andthe pressure signal P₂(t) on the line 32 is provided to a negative inputof the summer 44. The output of the summer 44 is provided on a line 45indicative of the difference between the two pressure signals P₁,P₂(e.g., P₁−P₂=P_(as1)).

[0059] The pressure sensors 18,20 together with the summer 44 create aspatial filter 33. The line 45 is fed to bandpass filter 46, whichpasses a predetermined passband of frequencies and attenuatesfrequencies outside the passband. In accordance with the presentinvention, the passband of the filter 46 is set to filter out (orattenuate) the dc portion and the high frequency portion of the inputsignals and to pass the frequencies therebetween. For example, in aparticular embodiment passband filter 6 is set to pass frequencies fromabout 1 Hz to about 100 Hz, for a 3 inch ID pipe flowing water at 10ft/sec. Other passbands may be used in other embodiments, if desired.Passband filter 46 provides a filtered signal P_(asf) 1 on a line 48 toCross-Correlation Logic 50, described hereinafter.

[0060] The pressure signal P₃(t) on the line 34 is provided to apositive input of a summer 54 and the pressure signal P₄(t) on the line36 is provided to a negative input of the summer 54. The pressuresensors 22,24 together with the summer 54 create a spatial filter 35.The output of the summer 54 is provided on a line 55 indicative of thedifference between the two pressure signals P₃,P₄ (e.g., P₃−P₄=Pas₂).The line 55 is fed to a bandpass filter 56, similar to the bandpassfilter 46 discussed hereinbefore, which passes frequencies within thepassband and attenuates frequencies outside the passband. The filter 56provides a filtered signal P_(asf) 2 on a line 58 to theCross-Correlation Logic 50. The signs on the summers 44,54 may beswapped if desired, provided the signs of both summers 44,54 are swappedtogether. In addition, the pressure signals P₁,P₂,P₃,P₄ may be scaledprior to presentation to the summers 44,54.

[0061] The Cross-Correlation Logic 50 calculates a known time domaincross-correlation between the signals P_(asf) 1 and P_(asf) 2 on thelines 48,58, respectively, and provides an output signal on a line 60indicative of the time delay τ it takes for an vortical flow field 15(or vortex, stochastic, or vortical structure, field, disturbance orperturbation within the flow) to propagate from one sensing region 14 tothe other sensing region 16. Such vortical flow disturbances, as isknown, are coherent dynamic conditions that can occur in the flow whichsubstantially decay (by a predetermined amount) over a predetermineddistance (or coherence length) and convect (or flow) at or near theaverage velocity of the fluid flow. As described above, the vorticalflow field 15 also has a stochastic or vortical pressure disturbanceassociated with it. In general, the vortical flow disturbances 15 aredistributed throughout the flow, particularly in high shear regions,such as boundary layers (e.g., along the inner wall of the pipe 12) andare shown herein as discrete vortical flow fields 15. Because thevortical flow fields 15 (and the associated pressure disturbance)convect at or near the mean flow velocity, the propagation time delay τis related to the velocity of the flow by the distance ΔX between themeasurement regions 14,16, as discussed hereinafter.

[0062] Although pressure disturbances associated with vortical flowfields 15 occur naturally in most flow conditions, an optionalcircumferential groove 70 may be used in the inner diameter of the pipe12 to help generate unsteady flow fields in the form of vertices intothe flow. However, the groove 70 is not required for the presentinvention to operate, due to vortex generation which naturally occursalong the pipe inner wall, as discussed hereinbefore. Instead of asingle circumferential groove 70 a plurality of axially spacedcircumferential grooves may be used. The dimensions and geometry of thegroove(s) 70 may be set based on the expected flow conditions and otherfactors. The axial cross-sectional shape of the groove 70 may berectangular, square, triangular, circular, oval, star, or other shapes.Other techniques may be used as vortex generators if desired includingthose that may protrude within the inner diameter of pipe 12.

[0063] A spacing signal ΔX on a line 62 indicative of the distance ΔXbetween the sensing regions 14,16 is divided by the time delay signal τon the line 60 by a divider 64 which provides an output signal on theline 42 indicative of the convection velocity U_(c)(t) of the fluidflowing in the pipe 12, which is related to (or proportional to orapproximately equal to) the average (or mean) flow velocity U_(f)(t) ofthe fluid, as defined below:

U _(c)(t)=ΔX/τ∝U _(f)(t)  Eq. 1

[0064] The convection velocity U_(c)(t) may then be calibrated to moreprecisely determine the mean velocity U_(f)(t) if desired. The result ofsuch calibration may require multiplying the value of the convectionvelocity U_(c)(t) by a calibration constant (gain) and/or adding acalibration offset to obtain the mean flow velocity U_(f)(t) with thedesired accuracy. Other calibration may be used if desired. For someapplications, such calibration may not be required to meet the desiredaccuracy. The velocities U_(f)(t),U_(c)(t) may be converted tovolumetric flow rate by multiplying the velocity by the cross-sectionalarea of the pipe.

[0065] Referring to FIGS. 2,3,4, as is known, cross-correlation may beused to determine the time delay τ between two signals y₁(t),y₂(t)separated by a known distance ΔX, that are indicative of quantities 80that convect with the flow (e.g., density perturbations, concentrationperturbations, temperature perturbations, vortical pressuredisturbances, and other quantities). In FIG. 3, the signal y₂(t) lagsbehind the signal y₁(t) by 0.15 seconds. If a time domaincross-correlation is taken between the two signals y₁(t),y₂(t), theresult is shown in FIG. 4 as a curve 84. The highest peak 86 of thecurve 84 shows the best fit for the time lag τ between the two signalsy₁(t),y₂(t) is at 0.15 seconds which matches the reference time delayshown in FIG. 3.

[0066] Referring to FIG. 1, as discussed hereinbefore, since pressuredisturbances associated within the vortical flow field 15 convect (orflow) at or near the average velocity of the fluid flowing in the pipe12, the vortical pressure disturbances observed at the downstreamlocation 16 are substantially a time lagged version of the vorticalpressure disturbances observed at the upstream location 14. However, thetotal vortical pressure perturbations or disturbances in a pipe may beexpressed as being comprised of vortical pressure disturbances(P_(vortical)), acoustic pressure disturbances (P_(acoustic)) and othertypes of pressure disturbances (P_(other)) as shown below expressed interms of axial position along the pipe at any point in time:

P(x,t)=P_(vortical) (x,t)+P_(acoustic) (x,t)+P_(other) (x,t)  Eq. 2

[0067] As a result, the unsteady pressure disturbances P_(Vortical) canbe masked by the acoustic pressure disturbances P_(acoustic) and theother types of pressure disturbances P_(other). In particular, thepresence of the acoustic pressure disturbances that propagate bothupstream and downstream at the speed of sound in the fluid (sonicvelocity), can prohibit the direct measurement of velocity fromcross-correlation of direct vortical pressure measurements.

[0068] The present invention uses temporal and spatial filtering toprecondition the pressure signals to effectively filter out the acousticpressure disturbances P_(acoustic) and other long wavelength (comparedto the sensor spacing) pressure disturbances in the pipe 12 at the twosensing regions 14,16 and retain a substantial portion of the vorticalpressure disturbances P_(vortical) associated with the vortical flowfield 15 and any other short wavelength (compared to the sensor spacing)low frequency pressure disturbances P_(other). In accordance with thepresent invention, if the low frequency pressure disturbances P_(other)are small, they will not substantially impair the measurement accuracyof P_(vortical).

[0069] The P_(vortical) dominated signals from the two regions 14,16 arethen cross-correlated to determine the time delay τ between the twosensing locations 14,16. More specifically, at the sensing region 14,the difference between the two pressure sensors 18,20 creates a spatialfilter 33 that effectively filters out (or attenuates) acousticdisturbances for which the wavelength λ of the acoustic wavespropagating along the fluid is long (e.g., ten-to-one) compared to thespacing X₁ between the sensors. Other wavelength to sensor spacingratios may be used to characterize the filtering, provided thewavelength to sensor spacing ratio is sufficient to satisfy thetwo-to-one spatial aliasing Nyquist criteria.

[0070] Thus, if the pressure sensors P₁,P₂ have an axial spacing X₁, andassuming that the spatial filter 33 will attenuate acoustic wavelengthslonger than about 10 times the sensor spacing X₁, the smallest acousticwavelength λmin that is attenuated would be:

λ_(min)=10(X₁)  Eq. 3

[0071] One dimensional acoustic disturbances are also governed by thefollowing known inverse wavelength-frequency relation:

λ=a/f or f=a/λ  Eq. 4

[0072] where a is the speed of sound of the fluid, f is the frequency ofthe acoustic disturbance, and λ is the wavelength of the acousticdisturbance.

[0073] Using Eq. 4, such a spatial filter would filter out frequenciesbelow about:

f _(max) =a/λ _(min)   Eq. 5

[0074] For example, using water (a=5,000 ft/sec) with a sensor spacingX₁=3 inches, the above described spatial acoustic filtering would filterout acoustic frequencies up to a maximum frequency of about 2000 Hz (or5,000*12/30). Thus, the acoustic frequency content of the output signalP_(as1) of the spatial filter 33 (i.e., differenced vortical pressuresignal) will be effectively removed for frequencies below about 2000 Hzand wavelengths above 30 inches (using Eq. 3). The above discussion onthe spatial filter 33 also applies to the second spatial filter 35comprising the other pair of pressure signals P₃,P₄, axially spaced adistance X₂ apart, which provides the differenced vortical pressuresignal P_(as2).

[0075] Referring to FIG. 5, relevant features of the power spectraldensity (PSD) of typical vortical pressure disturbances P_(vortical) isshown by a curve 90 that has a flat region (or bandwidth) up to afrequency F_(v) and then decreases with increasing frequency f. Thevalue of F_(v) is approximately equal to U/r, where U is the flow rateand r is the radius of the pipe. For example, for a flow rate U of about10 ft/sec and a pipe radius r of about 0.125 ft (or about 1.5 inches),the bandwidth F_(v) of the vortical pressure disturbances P_(vortical)would be about 80 Hz (10/0.125). The PSD of the acoustic pressuredisturbances P_(acoustic) has a profile that is determined by theenvironment and other factors and is indicated in the figure by anarbitrary curve 91, and typically has both low and high frequencycomponents.

[0076] Referring to FIG. 6, in general, the acoustic pressuredisturbances P_(acoustic) have an inverse wavelength-frequencyrelationship as shown in Eq. 4, which has long wavelengths at lowfrequencies and short wavelengths at high frequencies as indicated bythe regions 96,98, respectively. Conversely, the vortical pressuredisturbances P_(vortical) have both long and short wavelengths asindicated by the regions 96,97, respectively; however, they existprimarily at low frequencies (as discussed hereinbefore with referenceto FIG. 5). Thus, both P_(acoustic) and P_(vortical) exist in the longwavelength, low frequency region 96, and only P_(vortical) exists in theshort wavelength low frequency region 97.

[0077] The acoustic spatial filters 33,35 (FIG. 1) discussedhereinbefore block or attenuate wavelengths longer than λ_(as) andfrequencies below f_(as), as indicated by the region 96. Also, thebandpass filters (BPF) 46,56 (FIG. 1) block or attenuate highfrequencies above f_(pb) having short and long wavelengths as indicatedby a region 102 and pass frequencies below f_(as) where the P_(vortical)signals exist. Thus, after the spatial filters 33,35 and the BPF's46,56, the resultant filtered signals P_(asf) 1, P_(asf) 2 on the lines48,58 (FIG. 1) will be dominated by the short wavelength unsteadypressure disturbances P_(vortical) as indicated by the region 97 (FIG.6) at frequencies below f_(pb) and as indicated by a portion 94 of thecurve 90 in the BPF passband 95 (FIG. 5).

[0078] Accordingly, referring to FIG. 5, the spatial filters 33,35(FIG. 1) block the long wavelengths, which, for the acoustic pressuredisturbances P_(acoustic), occur at low frequencies as indicated to theleft of a dashed line 92 at frequencies below the frequency f_(as). Adashed line 93 indicates the attenuation of the acoustic pressureP_(acoustic) signal 91 below the frequency f_(as) at the output of thespatial filters. The vortical pressure disturbances P_(vortical) aresubstantially not attenuated (or only slightly attenuated) becauseP_(vortical) has short wavelengths at low frequencies that aresubstantially passed by the spatial filter. The BPF's 46,56 (FIG. 1)block or attenuate frequencies outside the passband indicated by a rangeof frequencies 95, and passes the unsteady pressure disturbancesassociated with stochastic flow fields 15 (FIG. 1) within the passband95.

[0079] Alternatively, instead of the filters 46,56 being bandpassfilters, provided the dc content is acceptably small, the filters 46,56may comprise low pass filters, having a bandwidth similar to the upperband of the high pass filters discussed hereinbefore. If a low passfilter is used as the filters 46,56, the passband is shown as a range offrequencies 89. It should be understood that the filters 46,56 are notrequired for the present invention if the PSD of the acoustic pressuredisturbances P_(acoustic) has substantially no or low PSD energy contentin frequencies above the stopband of the spatial filter that does notadversely affect the measurement accuracy.

[0080] Referring to FIGS. 7 and 1, for the four ac pressure sensors18,20,22,24 evenly axially spaced at 1 inch apart (X₁, X₂) along thepipe 12, and providing ac pressure signals P₁,P₂,P₃,P₄, respectively,the frequency power spectrum for P₁ and P₂ are shown by curves 100,102,respectively, for water flowing in an horizontal flow loop at a velocityof 11.2 ft/sec in a 2 inch diameter schedule 80 pipe using conventionalpiezoelectric ac pressure transducers. The power spectra of the curves100,102 are nearly identical. The power spectrum of the differenceP_(as1) between the two signals P₁,P₂, shown by a curve 104 is reducedin certain frequency bands (e.g., 100-150 Hz) and increased in otherfrequency bands (e.g., 200-250 Hz) as compared to the individual signals100,102.

[0081] Referring to FIGS. 8 and 1, the cross correlation between thesignals P_(as1). (or P₁-P₂) and P_(as2) (P₃-P₄) is shown as a curve 110.The highest peak 112 indicates the best fit for the time lag between thetwo signals P_(as1), P_(as2) as 0.015 seconds. Because the four sensorsP₁ to P₄ were evenly axially spaced 1 inch apart, the effective distanceΔX between the sensor pairs is 2 inches. Thus, the velocity measuredfrom Eq. 1 is 11.1 ft/sec (2/12/0.015) using the present invention andthe actual velocity was 11.2 ft/sec.

[0082] Referring to FIG. 9, for the configuration described with FIGS.1,7,8 above, the velocity was measured at various flow rates and plottedagainst a reference velocity value. A solid line 120 shows the referencevelocity, the triangles 122 are the measured data, and a line 124 is acurve fit of the data 122. This illustrates that the present inventionpredicts the flow velocity within a pipe (or conduit).

[0083] The pressure sensors 18,20,22,24 described herein may be any typeof pressure sensor, capable of measuring the unsteady (or ac or dynamic)pressures within a pipe, such as piezoelectric, optical, capacitive,piezo-resistive (e.g., Wheatstone bridge), accelerometers, velocitymeasuring devices, displacement measuring devices, etc. If opticalpressure sensors are used, the sensors 18-24 may be Bragg grating basedpressure sensors, such as that described in copending U.S. patentapplication, Ser. No. 08/925,598, entitled “High Sensitivity Fiber OpticPressure Sensor For Use In Harsh Environments”, filed Sep. 8, 1997, nowU.S. Pat. No. 6,016,702. Alternatively, the sensors 18-24 may beelectrical or optical strain gages attached to or embedded in the outeror inner wall of the pipe which measure pipe wall strain, includingmicrophones, hydrophones, or any other sensor capable of measuring theunsteady pressures within the pipe 12. In an embodiment of the presentinvention that utilizes fiber optics as the pressure sensors 18-24, theymay be connected individually or may be multiplexed along one or moreoptical fibers using wavelength division multiplexing (WDM), timedivision multiplexing (TDM), or any other optical multiplexingtechniques (discussed more hereinafter).

[0084] Such harsh environments are typically found in the industrialprocess area and include sensor exposure to acids, caustics, nuclearenergy, electromagnetic interference and exposure to explosive vaporsamong other hazards. Because the sensor is glass based it is chemicallyimpervious to most industrial process related chemicals. Further becausethe sensor of the present invention uses light for signal transmissionit does not require any electrical power and as such is not influencedby electromagnetic fields and cannot create arcing or explosions whenused in the presence of flammable vapors. In addition the sensor of thepresent invention has no moving parts, such as a bellows, which makesthe device more reliable and less susceptible to system hysteresis foundin other mechanical pressure sensors that utilize diaphragms bellows orother displacement type devices.

[0085] Referring to FIG. 13, if a strain gage is used as one or more ofthe pressure sensors 18-24 (FIGS. 14-20), it may measure the unsteady(or dynamic or ac) pressure variations P_(in) inside the pipe 12 bymeasuring the elastic expansion and contraction, as represented byarrows 350, of the diameter (and thus the circumference as representedby arrows 351) of the pipe 12. In general, the strain gages wouldmeasure the pipe wall deflection in any direction in response tounsteady pressure signals inside the pipe 12. The elastic expansion andcontraction of pipe 12 is measured at the location of the strain gage asthe internal pressure P_(in) changes, and thus measures the local strain(axial strain, hoop strain or off axis strain), caused by deflections inthe directions indicated by arrows 351, on the pipe 12. The amount ofchange in the circumference is variously determined by the hoop strengthof the pipe 12, the internal pressure P_(in), the external pressureP_(out) outside the pipe 12, the thickness T_(w) of the pipe wall 352,and the rigidity or modulus of the pipe material. Thus, the thickness ofthe pipe wall 352 and the pipe material in the sensor sections 14,16(FIG. 1) may be set based on the desired sensitivity of filter 33 andother factors and may be different from the wall thickness or materialof the pipe 12 outside the sensing regions 14,16.

[0086] Still with reference to FIG. 13 and FIG. 1, if an accelerometeris used as one or more of the pressure sensors 18-24 (FIGS. 14-20), itmay measure the unsteady (or dynamic or ac) pressure variations P_(in)inside the pipe 12 by measuring the acceleration of the surface of pipe12 in a radial direction, as represented by arrows 350. The accelerationof the surface of pipe 12 is measured at the location of theaccelerometer as the internal pressure P_(in) changes and thus measuresthe local elastic dynamic radial response of the wall 352 of the pipe.The magnitude of the acceleration is variously determined by the hoopstrength of the pipe 12, the internal pressure P_(in), the externalpressure P_(out) outside the pipe 12, the thickness T_(w) of the pipewall 352, and the rigidity or modulus of the pipe material. Thus, thethickness of the pipe wall 352 and the pipe material in the sensorsections 14,16 (FIG. 1) may be set based on the desired sensitivity offilter 33 and other factors and may be different from the wall thicknessor material of the pipe 12 outside the sensing region 14. Alternatively,the pressure sensors 18-24 may comprise a radial velocity ordisplacement measurement device capable of measuring the radialdisplacement characteristics of wall 352 of pipe 12 in response topressure changes caused by unsteady pressure signals in the pipe 12.

[0087] Referring to FIGS. 14,15,16, if an optical strain gage is used,the ac pressure sensors 18-24 may be configured using an optical fiber300 that is coiled or wrapped around and attached to the pipe 12 at eachof the pressure sensor locations as indicated by the coils or wraps302,304,306,308 for the pressures P₁,P₂,P₃,P₄, respectively. The fiberwraps 302-308 are wrapped around the pipe 12 such that the length ofeach of the fiber wraps 302-308 changes with changes in the pipe hoopstrain in response to unsteady pressure variations within the pipe 12and thus internal pipe pressure is measured at the respective axiallocation. Such fiber length changes are measured using known opticalmeasurement techniques as discussed hereinafter. Each of the wrapsmeasures substantially the circumferentially averaged pressure withinthe pipe 12 at a corresponding axial location on the pipe 12. Also, thewraps provide axially averaged pressure over the axial length of a givenwrap. While the structure of the pipe 12 provides some spatial filteringof short wavelength disturbances, we have found that the basic principleof operation of the invention remains substantially the same as that forthe point sensors described hereinbefore.

[0088] Referring to FIG. 14, for embodiments of the present inventionwhere the wraps 302,304,306,308 are connected in series, pairs of Bragggratings (310,312), (314,316), (318,320), (322,324) may be located alongthe fiber 300 at opposite ends of each of the wraps 302,304,306,308,respectively. The grating pairs are used to multiplex the pressuresignals P₁,P₂,P₃,P₄ to identify the individual wraps from optical returnsignals. The first pair of gratings 310,312 around the wrap 302 may havea common reflection wavelength λ₁, and the second pair of gratings314,316 around the wrap 304 may have a common reflection wavelength λ₂,but different from that of the first pair of gratings 310,312.Similarly, the third pair of gratings 318,320 around the wrap 306 have acommon reflection wavelength λ₃, which is different from λ₁,λ₂, and thefourth pair of gratings 322,324 around the wrap 308 have a commonreflection wavelength λ₄, which is different from λ₁,λ₂,λ₃.

[0089] Referring to FIG. 15, instead of having a different pair ofreflection wavelengths associated with each wrap, a series of Bragggratings 360-368 with only one grating between each of the wraps 302-308may be used each having a common reflection wavelength λ₁.

[0090] Referring to FIGS. 14 and 15 the wraps 302-308 with the gratings310-324 (FIG. 14) or with the gratings 360-368 (FIG. 15) may beconfigured in numerous known ways to precisely measure the fiber lengthor change in fiber length, such as an interferometric, Fabry Perot,time-of-flight, or other known arrangements. One example oftime-of-flight (or Time-Division-Multiplexing; TDM) would be where anoptical pulse having a wavelength is launched down the fiber 300 and aseries of optical pulses are reflected back along the fiber 300. Thelength of each wrap can then be determined by the time delay betweeneach return pulse.

[0091] While the gratings 310-324 are shown oriented axially withrespect to pipe 12, in FIGS. 14,15, they may be oriented along the pipe12 axially, circumferentially, or in any other orientations. Dependingon the orientation, the grating may measure deformations in the pipewall 352 with varying levels of sensitivity. If the grating reflectionwavelength varies with internal pressure changes, such variation may bedesired for certain configurations (e.g., fiber lasers) or may becompensated for in the optical instrumentation for other configurations,e.g., by allowing for a predetermined range in reflection wavelengthshift for each pair of gratings. Alternatively, instead of each of thewraps being connected in series, they may be connected in parallel,e.g., by using optical couplers (not shown) prior to each of the wraps,each coupled to the common fiber 300.

[0092] Referring to FIG. 16, alternatively, the sensors 18-24 may alsobe formed as individual non-multiplexed interferometric sensor bywrapping the pipe 12 with the wraps 302-308 without using Bragg gratingswhere separate fibers 330,332,334,336 may be fed to the separate wraps302,304,306,308, respectively. In this particular embodiment, knowninterferometric techniques may be used to determine the length or changein length of the fiber 10 around the pipe 12 due to pressure changes,such as Mach Zehnder or Michaelson Interferometric techniques such asthose set forth in U.S. patent application Ser. No. 09/726,059, titled“Method and Apparatus for Interrogating Fiber Optic Sensors ” filed Nov.29, 2000.

[0093] The interferometric wraps may be multiplexed such as is describedin Dandridge, et al, “Fiber Optic Sensors for Navy Applications”, IEEE,Feb. 1991, or Dandridge, et al, “Multiplexed Interferometric FiberSensor Arrays”, SPIE, Vol. 1586, 1991, pp176-183. Other techniques todetermine the change in fiber length may be used. Also, referenceoptical coils (not shown) may be used for certain interferometricapproaches and may also be located on or around the pipe 12 but may bedesigned to be insensitive to pressure variations.

[0094] Referring to FIGS. 17 and 18, instead of the wraps 302-308 beingoptical fiber coils wrapped completely around the pipe 12, the wraps302-308 may have alternative geometries, such as a “radiator coil”geometry (FIG. 17) or a “race-track” geometry (FIG. 18), which are shownin a side view as if the pipe 12 is cut axially and laid flat. In thisparticular embodiment, the wraps 302-208 are not necessarily wrapped 360degrees around the pipe, but may be disposed over a predeterminedportion of the circumference of the pipe 12, and have a length longenough to optically detect the changes to the pipe circumference. Othergeometries for the wraps may be used if desired. Also, for any geometryof the wraps described herein, more than one layer of fiber may be useddepending on the overall fiber length desired. The desired axial lengthof any particular wrap is set depending on the characteristics of the acpressure desired to be measured, for example the axial length of thepressure disturbance caused by a vortex to be measured.

[0095] Referring to FIGS. 19 and 20, embodiments of the presentinvention include configurations wherein instead of using the wraps302-308, the fiber 300 may have shorter sections that are disposedaround at least a portion of the circumference of the pipe 12 that canoptically detect changes to the pipe circumference. It is further withinthe scope of the present invention that sensors may comprise an opticalfiber 300 disposed in a helical pattern (not shown) about pipe 12. Asdiscussed herein above, the orientation of the strain sensing elementwill vary the sensitivity to deflections in pipe wall 352 caused byunsteady pressure transients in the pipe 12.

[0096] Referring to FIG. 19, in particular, the pairs of Bragg gratings(310,312), (314,316), (318,320), (322,324) are located along the fiber300 with sections 380-386 of the fiber 300 between each of the gratingpairs, respectively. In that case, known Fabry Perot (resonator, cavity,interferometer or other known Fabry Perot arrangement), interferometric,time-off-light or fiber laser sensing techniques may be used to measurethe strain in the pipe.

[0097] Referring to FIG. 20, alternatively, individual gratings 370-376may be disposed on the pipe and used to sense the unsteady variations instrain in the pipe 12 (and thus the unsteady pressure within the pipe)at the sensing locations. When a single grating is used per sensor, thegrating reflection wavelength shift will be indicative of changes inpipe diameter and thus pressure.

[0098] Any other technique or configuration for an optical strain gagemay be used. The type of optical strain gage technique and opticalsignal analysis approach is not critical to the present invention, andthe scope of the invention is not intended to be limited to anyparticular technique or approach.

[0099] For any of the embodiments described herein, the pressuresensors, including electrical strain gages, optical fibers and/orgratings among others as described herein, may be attached to the pipeby adhesive, glue, epoxy, tape or other suitable attachment means toensure suitable contact between the sensor and the pipe 12. The sensorsmay alternatively be removable or permanently attached via knownmechanical techniques such as mechanical fastener, spring loaded,clamped, clamshell arrangement, strapping or other equivalents.Alternatively, the strain gages, including optical fibers and/orgratings, may be embedded in a composite pipe. If desired, for certainapplications, the gratings may be detached from (or strain oracoustically isolated from) the pipe 12 if desired. It is also withinthe scope of the present invention that any other strain sensingtechnique may be used to measure the variations in strain in the pipe,such as highly sensitive piezoresistive, electronic or electric, straingages attached to or embedded in the pipe 12. Referring to FIG. 29different known configurations of highly sensitive piezoresistive straingages are shown and may comprise foil type gages. Referring to FIG. 30an embodiment of the present invention is shown wherein pressure sensors18, 20, comprise strain gages 203. In this particular embodiment straingages 203 are disposed about a predetermined portion of thecircumference of pipe 12. The axial placement of and separation distanceX₁ between pressure sensors 18, 20 are determined as describedhereinabove. In particular, the placement is dependent upon thecharacteristics of the ac pressure desired to be measured, for examplethe spatial (axial or transverse) length and coherence length of thepressure disturbance caused by the vortex, or unsteady pressuredisturbance, to be measured.

[0100] Referring to FIG. 10, there is shown an embodiment of the presentinvention comprising three spatial filters, 33,35,37 each comprising apair of pressure sensors measuring at total of six unsteady pressuresP₁-P₆ (three pairs), each pressure sensor being a plurality (e.g., 10meters) of fiber optic wraps and the sensors being evenly axially spacedat 1.8 inches apart, on a pipe having an inner diameter of 3.0 inches, awall thickness of 0.22 inches and made of J55 steel production tubing,is shown. The three spatial filters, 33,35,37 provide spatially filteredac pressure signals P_(as1), P_(as2), P_(as3), respectively.

[0101] These ac pressure signals P_(sa1), P_(as2), P_(as3) may be usedas input to a variety of devices and used as desired. It is within thescope of the present invention that any number of spatial filters andspatial time filters may be used and that the particular embodiment willdictate the quantity and the spacing (not shown in FIG. 10) between eachspatial filter. It is noted that although pressure sensors P₁-P₆ areshown as point sensors it is within the scope of the present inventionthat the sensors comprise any configuration capable of accuratelydetecting pressure changes in pipe 12 including fiber optic wraps asdescribed herein.

[0102] Referring to FIG. 11, for the configuration of FIG. 10, and for aliquid flow mixture of 100% oil at 111.2 gal/minute (or about 5.05ft/sec for a 3 inch pipe), the invention provides cross-correlationcurves 130,132,134. The curves 130,132,134 correspond to velocities of5.538 ft/sec, 5.541 ft/sec, 5.5822 ft/sec, for the cross-correlationbetween P_(as1) and P_(as2) (vela), P_(as2) and P_(as3) (vel-b), andP_(as1) and P_(as3) (vel-c), for the groups of sensors a,b,c,respectively, shown in FIG. 10. Referring to FIG. 12, it is shown thatthe present invention will work over a wide range of oil/water mixtures.In particular, the first two pairs of sensors (P₁,P₂, and P₃,P₄) of FIG.10 measured the velocity at various flow rates against a referencevelocity value. Data points 151-162 are groupings of measured velocitydata points derived from the sensors (P₁,P₂,P₃,P₄) and are plottedagainst a reference velocity line 150. The data points 151-162 also showthe invention will work for fluid flowing in either direction in thepipe 12. The negative flow data points 151-158 were taken with a fluidof 100% oil and the positive flow data points 159-162 were taken over arange of various oil/water mixtures. Specifically, data points 159represent 100 individual data points taken at velocities from about 5.2ft/sec to about 5.7 ft/sec and in oil/water mixtures from 0% to 100%water. Data point 160 represents a single individual data point taken ata velocity of about 9.9 ft/sec in an oil/water mixture of 0% water. Datapoint 161 represents a single individual data point taken at velocity ofabout 13.7 ft/sec in an oil/water mixture of 0% water. Similarly, datapoints 162 represent 21 individual data points taken at velocities fromabout 18.0 ft/sec to about 19.0 ft/sec and in oil/water mixtures from 0%to 100% water. The departure of the raw data 151-162 from the referencevelocity line 150 is caused, in part, by the fact that the points werenot calibrated and that the reference velocity at each point was takenmanually by a technician. Had the data points been calibrated andelectronically matched to the sensed points the departure from thereference line 150 would not have been as large as depicted in thefigure.

[0103] The present invention will also work over a wide range ofmultiphase fluid mixtures. Also, the invention will work for very lowflow velocities, e.g., at or below 1 ft/sec (or about 20.03 gal/min, ina 3 inch diameter ID pipe) and has no maximum flow rate limit. Further,the invention will work with the pipe 12 being oriented vertical,horizontal, or any other orientation. Also the invention will workequally well independent of the direction of the flow along the pipe 12.

[0104] Referring to FIG. 21, instead of the unsteady pressure sensors18,20 and the corresponding unsteady pressure signals P₁,P₂ being spacedaxially along the pipe 12, the sensors 18,20 may be spacedcircumferentially apart at substantially the same axial location. Inthat case, the spatial filter 33(FIG. 1) (i.e., the difference betweenthe two signals P₁, P₂, P₃,P₄) filters out substantially all onedimensional acoustic waves propagating through the sensing region 14.Alternatively, referring to FIG. 22, instead of the pressure sensors18,20 being located directly across from each other, the signal P₂ maybe measured at a distance circumferentially closer to the sensor 18, asindicated by a sensor 200. The circumferential distance 53 between thetwo sensors 18,200 should be large enough to independently measure apropagating vortical pressure field 15 such that the spatial filter 33output is not zero for the measured vortex 15, i.e., that thecircumferential distance 53 is greater than the transverse spatiallength of vortex 15. In addition, the distance X₁ (FIG. 1) should beless than or equal to the axial coherence length of the vortex 15 suchthat the spatial filter output is indicative of a measured vortex 15.

[0105] The thickness and rigidity of the outer wall of the pipe 12 isrelated to the acceptable spacing X₁ (FIG. 1) between the sensors 18,20of the spatial filter 33. More specifically, the thinner or less rigidthe pipe 12 wall, the closer the sensors 18,20 can be to each other.

[0106] Also, for optimal performance, the distance X₁ between the twosensors 18,20 should be larger than the spatial length of the vorticalpressure field 15 such that each of the sensors 18,20 can independentlymeasure the propagating vortical pressure field 15 between the sensors18,20 at different times (such that the spatial filter 33 output is notzero for the measured vortex 15). Also, the distance X₁ should be withinthe coherence length of the vortex 15 such that the spatial filteroutput is indicative of a measured vortex 15. Also, for optimalperformance, the overall length L₁ between the first sensor 18 and thelast sensor 24 of the velocity sensing section should be within thecoherence length of the vortices 15 desired to be measured. Thecoherence length of the vortical flow field 15 is the length over whichthe vortical flow field remains substantially coherent, which is relatedto and scales with the diameter of the pipe 12.

[0107] Vortices that are sensed by only one of the spatial filters,because either a vortex is generated between the spatial filters orgenerated outside the spatial filters and decay between them, will besubstantially random events (in time and location) that will not becorrelated to the vortices that are sensed by and continuously occurringpast both spatial filters and, as such, will not significantly affectthe accuracy of the measurement.

[0108] Referring to FIG. 24, a particular embodiment of the presentinvention is shown therein where more than two sensors may be used forone or both of the spatial filters 33,35. In particular, the summer 44may have three inputs P₁,P₂,P₃, from three pressure sensors 220,222,224where the output signal P_(sa1)=P₁−2P₂+P₃. For optimal performance, theoverall axial length L of the filter 33 should be within the coherencelength of the vortices 15 being measured and the individual spacingbetween the sensors 220,222,224 should have the same criteria discussedhereinbefore for the spacing between two sensors 18,20.

[0109] Referring to FIG. 32, instead of using four pressure sensors tomake the spatial filters 33,35 three pressure sensors 600,602,604 may beused where the middle sensor 602 is used for both the spatial filters33,35.

[0110] Referring to FIGS. 28-31, instead of measuring the unsteadypressures P₁-P₄ on the exterior of the pipe 12, the invention will alsowork when the unsteady pressures are measured inside the pipe 12. Inparticular, the pressure sensors 18,20 that measure the pressures P₁,P₂,may be located anywhere within the pipe 12, having the same constraintsdiscussed hereinbefore for the exterior measurements. Any technique maybe used to measure the unsteady pressures inside the pipe 12.

[0111] Referring to FIGS. 25-27, the invention may also measure thevelocity of flow outside a pipe or tube 400. In that case, the tube 400may be placed within the pipe 12 and the pressures P₁-P₄ measured at theoutside of the tube 400. Any technique may be used to measure theunsteady pressures P₁-P₄ outside the tube 400.

[0112] Referring to FIG. 25, for example, the tube 400 may have theoptical wraps 302-308 wrapped around the tube 400 at each sensinglocation. Alternatively, any of the strain measurement or displacement,velocity or accelerometer sensors or techniques described herein may beused on the tube 400. Referring to FIG. 26, alternatively, the pressuresP₁-P₄ may be measured using direct pressure measurement sensors ortechniques described herein. Any other type of unsteady pressure sensors18-24 may be used to measure the unsteady pressures within the pipe 12.

[0113] Alternatively, referring to FIG. 27, hydrophones 402-408 may beused to sense the unsteady pressures within the pipe 12. In that case,the hydrophones 402-408 may be located in the tube 400 for ease ofdeployment or for other reasons. The hydrophones 402408 may be fiberoptic, electronic, piezoelectric or other types of hydrophones. If fiberoptic hydrophones are used, the hydrophones 402-408 may be connected inseries or parallel along the common optical fiber 300.

[0114] The tube 400 may be made of any material that allows the unsteadypressure sensors to measure the pressures P₁-P₄ and may be hollow,solid, or gas filled or fluid filled. One example of a dynamic pressuresensor is described in co-pending commonly-owned U.S. Pat. No.6,233,374, entitled “Mandrel Wound Fiber Optic Pressure Sensor”. Also,the end 422 of the tube 400 may be closed or open. If the end 422 isclosed, the flow path would be around the end 422 as indicated by lines424. If the end 422 is open, the flow path would be through the insideof the tube, as indicated by a line 426 and the pressure would bemeasured inside of the pipe 12. For harsh environment industrialapplications, the tube 400 may be coiled tubing having the pressuresensors for sensing P₁-P₄ inside the tubing 400.

[0115] Although the invention has been described with respect to thedetection of certain types of unsteady flow fields and the pressuredisturbances associated therewith, it should be understood that theinvention will also detect any unsteady stochastic flow field and itsassociated pressure field that propagates within the flow, provided thespatial filters have a separation within the acceptable coherence lengthof the flow field to be measured and the sensor spacing within eachspatial filter is longer than a characteristic spatial length of thedisturbance. Some examples of such other stochastic flow fields are gasbubbles, gas slugs, particles, or chunks of material, which may travelin the flow at different rates than the mean flow velocity, therebycreating a traveling pressure disturbance, which exhibits a velocityslip between it and the other constituents in the mixture.

[0116] Accordingly, the invention may be used to detect such differentflow rates within the same mixture (e.g., the flow rate of an unsteadypressure field within the mixture). Also, such unsteady flow fields,when traveling at different rates from other portions of the mixture,may also shed vortices in the flow that may propagate with the flow andbe detected as an unsteady flow field by the present invention.

[0117] Referring to FIG. 33, there is shown an embodiment of the presentinvention in a typical industrial processing application, the sensingsection 51 may be connected to or part of process tubing 502 (analogousto the pipe 12 in the test section 51) within an industrial processcontrol system 500. The isolation sleeve 410 may be located over thesensors 18, 20, 22, 24 as discussed hereinbefore and attached to thepipe 502 at the axial ends to protect the sensors 18, 20, 22, 24 (orfibers) from damage during deployment, use, or retrieval, and/or to helpisolate the sensors from acoustic external pressure effects that mayexist outside the pipe 502, and/or to help isolate ac pressures in thepipe 502 from ac pressures outside the pipe 502. The advantages andeffect of the isolation sleeve 410, as well as other isolationtechniques, are described in commonly owned copending U.S. patentapplication Ser. No. 09/344,070, entitled “Measurement of PropagatingAcoustic Waves in Compliant Pipes” incorporated herein by reference inits entirety. The sensors 18, 20, 22, 24 are connected to a cable 506which may comprise the optical fiber and is connected to atransceiver/converter 510 of the control system 500.

[0118] When optical sensors are used, the transceiver/converter 510 maybe used to receive and transmit optical signals to the sensors 18-24 andprovides output signals indicative of the pressure P₁-P₄ at the sensors18-24 on the lines 30-36, respectively. Also, the transceiver/converter510 may be part of the Velocity Logic 40. The transceiver/converter 510may be any device that performs the corresponding functions describedherein. In particular, the transceiver/converter 510 together with theoptical sensors described hereinbefore may use any type of opticalgrating-based measurement technique, e.g., scanning interferometric,scanning Fabry Perot, acousto-optic-tuned filter (AOTF), optical filter,time-of-flight, etc., having sufficient sensitivity to measure the acpressures within the pipe.

[0119] A plurality of the sensors 10 of FIG. 33 of the present inventionmay be connected to a common cable and multiplexed together using anyknown multiplexing technique by connecting end 511 to other sensors (notshown). For instance, it is contemplated that the various embodiments ofthe sensor 10 of the present invention include the capability beingmultiplexed as well as capable of communication with various protocolsand systems currently in use in the industrial sensing area. Forinstance, and with reference to FIG. 33 there is shown a portion of aprocess control system 500 incorporating a sensor 10 in accordance withthe present invention. Fluid velocity logic 40 communicates signal Uc(t)along line 42 to control device 70, a computer or micro-processor forexample, where the information may be used to control the fluid velocityin pipe 502 through known controls means such as a pump, valve,throttle, etc. (not shown). In certain embodiments of control system 500and with appropriate electro-optical conversion of the sensor returnsignal to a conventional 4-20 mA signal the signal can be combined withother control devices and sensors at control device 70 via separateelectrical lines. In this particular embodiment the communication fromthe fiber optic sensor is performed with a 4-20 MA analog signal, andthe open protocol HART®. (Highway Addressable Remote Transducer) digitalcommunications format. Similarly, communication from the fiber opticsensor 10 may also be performed with open and interoperable protocolFOUNDATION™ Fieldbus that provides a digital communication link amongintelligent field level and control devices via electrical lines. Thecontrol device 70 can be configured for use with other processprotocols, including Device Bus, Sensor Bus, Profibus, the ethernet, andothers in use throughout the world. The use of feedthroughs 511, asshown in FIG. 33, make the sensor 10 of the present invention uniquelyqualified for industrial applications requiring multiple sensors. Theuse of sensors having feedthroughs in a large multi-point processenables connectivity to the multiple sensors through a single fiberoptic cable. Electronic sensors of the prior art require dedicatedwiring to the sensor and back to the instrumentation. For instance, atypical industrial process control system that utilizes electronicflowmeters of the prior art requires an electrical process loop tofacilitate both a power signal to the transmitters and bi-directionalcommunication, and can be constructed in accordance with a number of theaforementioned process communication protocols.

[0120] In operation, industrial process uses for the present inventioninclude reverse osmosis, coking, general refining uses, in-line pressuresensors for emissions monitoring, sensors for monitoring hydrogen,combustion control, gas composition analysis, distributed sensors intank gauging, multi-phase computational fluid dynamics, instrumentationof multiphase flows, among others.

[0121] It should be understood that any of the features,characteristics, alternatives or modifications described regarding aparticular embodiment herein may also be applied, used, or incorporatedwith any other embodiment described herein.

[0122] Although the invention has been described and illustrated withrespect to exemplary embodiments thereof, the foregoing and variousother additions and omissions may be made therein and thereto withoutdeparting from the spirit and scope of the present invention.

What is claimed is:
 1. An apparatus for use in an industrial process for measuring a velocity of a fluid moving in a pipe, comprising: a first filter which measures a vortical pressure field at a first axial location along the pipe and provides a first pressure signal indicative of said vortical pressure field; and a second filter which measures said vortical pressure field at a second axial location along the pipe and provides a second pressure signal indicative of said vortical pressure field; a signal processor, responsive to said first and said second pressure signals, which provides a velocity signal indicative of a velocity of the said vortical pressure field moving in the pipe.
 2. The apparatus of claim 1 wherein said velocity signal is related to a velocity of said fluid moving in said pipe.
 3. The apparatus of claim 1 wherein said velocity signal is indicative of the velocity of said fluid moving in said pipe.
 4. The apparatus of claim 1 further comprising a volumetric flow meter wherein said signal processor provides a flow signal indicative of the volumetric flow rate of said fluid flowing in said pipe.
 5. The apparatus of claim 1, wherein said first and said second filters filter out wavelengths associated with an acoustic pressure field and passes wavelengths associated with said vortical pressure field.
 6. The apparatus of claim 5, wherein said first filter comprises a first spatial filter; and said second filter comprises a second spatial filter.
 7. The apparatus of claim 6, wherein: said first spatial filter comprises at least a first and a second unsteady pressure sensors disposed a predetermined first distance apart from each other; and said second spatial filter comprises at least a third and a fourth unsteady pressure sensors disposed a predetermined second distance apart from each other.
 8. The apparatus of claim 7 wherein said at least one of said pressure sensors comprises a fiber optic pressure sensor.
 9. The apparatus of claim 1 wherein said signal processor comprises logic which calculates a cross-correlation between said first and said second inhomogeneous pressure signals and provides a time delay signal indicative of the time it takes for said vortical pressure field to move from said first location to said second location.
 10. The apparatus of claim 9 wherein said signal processor comprises logic responsive to said time delay signal which provides an inhomogeneous velocity signal indicative of the velocity of said vortical pressure field moving in said pipe.
 11. The apparatus of claim 9 wherein said signal processor comprises logic responsive to said time delay signal which provides said velocity signal indicative of the velocity of said fluid moving in said pipe.
 12. A method for use in an industrial process for measuring a velocity of a fluid moving in a pipe, the method comprising: a) measuring a vortical pressure field at a first location along the pipe and providing a first vortical pressure signal indicative of said vortical pressure field; b) measuring said vortical pressure field at a second location along the pipe and providing a second vortical pressure signal indicative of said vortical pressure field, said first and said second locations being an axial distance apart; and c) calculating the velocity using said first and said second vortical pressure signals.
 13. The method of claim 12, wherein said calculating step (c) comprises: d) calculating a cross-correlation of said first and said second pressure signals to obtain a time delay signal indicative of the time it takes for said vortical pressure field to move from said first location to said second location.
 14. The method of claim 13, wherein said calculating step (d) comprises: e) calculating a velocity signal from said time delay signal.
 15. The method of claim 14, wherein said calculating step (e) comprises: f) dividing said axial distance between said measurement locations by said time delay signal.
 16. The method of claim 12 wherein: said measuring step (a) comprises: measuring a first unsteady pressure and a second unsteady pressure; subtracting said second unsteady pressure from said first unsteady pressure to form said first vortical pressure signal; and said measuring step (b) comprises: measuring a third unsteady pressure and a fourth unsteady pressure; and subtracting said fourth unsteady pressure from said third unsteady pressure to form said second vortical pressure signal.
 17. The method of claim 12 wherein: said first vortical pressure signal is indicative of wavelengths associated with a vortical pressure field and not associated with an acoustic pressure field at said first location; and said second vortical pressure signal is indicative of wavelengths associated with said vortical pressure field and not associated with an acoustic pressure field at said second location.
 18. An industrial process control system for controlling a velocity of a fluid in an industrial fluid process, said system comprising: a pipe having at least one fluid flowing therein; a first filter which measures a vortical pressure field at a first axial location along the pipe and provides a first pressure signal indicative of said vortical pressure field; and a second filter which measures said vortical pressure field at a second axial location along the pipe and provides a second pressure signal indicative of said vortical pressure field; a signal processor, responsive to said first and said second pressure signals, which provides a velocity signal indicative of a velocity of the said vortical pressure field moving in the pipe; and a control device receiving said velocity signal and capable of controlling said velocity of said fluid to a predetermined level. 